Cantwell was born in Long Beach, California and moved to Pahrump, Nevada at the age of 10. After being expelled middle school and high school because of fighting, Cantwell began training in kickboxing at the age of 16, under the tutelage of Nick Blumgren at One Kick's Gym.[2][3]

Later that year, Cantwell signed with the UFC, which is owned by the WEC's parent company Zuffa. Cantwell faced fellow UFC newcomer Razak Al-Hassan at UFC: Fight for the Troops an earned a technical submission victory by dislocating Al-Hassan's elbow in an armbar, causing referee Mario Yamasaki to stop the fight.[5] Cantwell's celebration of the victory and post-fight comments sparked some criticism when he said, "I've been waiting so long to do that, you have no idea".[6] He later apologized for his behavior.[7]

Cantwell was expected to face Vladimir Matyushenko on January 2, 2010 at UFC 108,[8] but pulled out of the bout for undisclosed reasons.[9] Since there was no time to find a suitable replacement to face Matyushenko, the bout was called off.[10] UFC President Dana White mentioned at the UFC 108 Pre-Fight Press Conference that Cantwell may never fight again, but refused to disclose what Cantwell's medical issues were.[11]

With the questions surrounding his medical condition apparently cleared,[12] Cantwell was expected to face UFC newcomer Ricardo Romero on July 3, 2010 at UFC 116.[13] However, for unknown reasons, Cantwell was replaced by Seth Petruzelli.[14]

Cantwell was expected to face Stanislav Nedkov on October 16, 2010 at UFC 120.[15] However less than 48 hours before the event, Cantwell was forced off the card after suffering a knee injury during a pre-fight workout. With no time to find a suitable opponent for Nedkov, the bout was cancelled.[16]